The invention relates to an air spring strut for mounting between a vehicle axle and a vehicle body which includes a shock absorber and an air spring and has the following features:
(a) the shock absorber includes a cylinder and a piston which is attached to a piston rod and can be pushed into the cylinder. The foot point of the cylinder can be articulately attached to a wheel-guiding component and the upper end of the piston rod can be pivotally connected to a cover of the air spring;
(b) the air spring contains a resilient member which is clamped between a cover and a roll-off piston and forms a rolling lobe. The cover can be attached to the body of the vehicle and the roll-off piston is supported on the cylinder of the shock absorber; and,
(c) the outer wall of the resilient member of the air spring is at least partially enclosed by a bell.
Air spring struts having a shock absorber and an air spring are built into motor vehicles and function to suspend a vehicle axle with respect to the vehicle body and function to dampen the vibrations introduced into the vehicle body. A pivot movement of the air spring strut takes place when there is a deflection of a vehicle wheel relative to the vehicle body. Because of this pivot movement of the air spring strut, the air spring can be deformed in such a manner that transverse forces are applied by the air spring to the air spring strut. These transverse forces must be taken up by the shock absorber and can lead to an increased friction between the piston rod or piston and the cylinder of the shock absorber. As a consequence of the increased friction, a premature wear of the seals of the shock absorber can occur which seal the piston rod relative to the cylinder of the shock absorber. Suggestions have been made as to how the transverse forces in the air spring can be reduced with a pivot movement of the air spring strut.
An air spring strut of the kind referred to initially herein is disclosed in U.S. Pat. No. 5,667,203. In this air spring strut, the roll-off piston of the air spring is mounted so as to be wobbly movable on the cylinder of the shock absorber. For a pivot movement of the air spring strut, the wobbly movable arrangement of the roll-off piston makes possible a pivot movement of the roll-off piston relative to the cylinder of the shock absorber. This pivot movement leads to the situation that the resilient member of the air spring forms an almost rotationally symmetrical roll-off lobe in every deflection condition of the air spring strut. The resilient member of the air spring is surrounded by a bell. For this reason, the transverse forces occurring in the air spring and the transverse forces introduced into the shock absorber by the air spring are reduced.
However, in the air spring strut disclosed in U.S. Pat. No. 5,667,203, it can happen that there is contact between the roll-off piston and the part of the resilient member of the air spring which is braced on the bell of the air spring when there is a large pivot movement of the roll-off piston of the air spring relative to the cylinder of the shock absorber. The contact can lead to damage of the resilient member of the air spring. This is especially then the case when the air spring piston plunges deep into the air spring. A further disadvantage of the air spring strut known from the above publication is that the roll-off piston of the air spring must reach far up to the lower end of the cylinder of the shock absorber so that pivot movements between the roll-off piston and the cylinder are possible without an angular offset of the roll-off piston which is too great (a large angle offset between the roll-off piston and the cylinder would be disadvantageous because a collision could occur between the roll-off piston and the cylinder or between the roll-off piston and the bell). In the lower region of the air spring strut, this can lead to problems of space for equipment especially when other components such as sensors, et cetera, are intended to be provided in this region.
It is an object of the invention to provide an air spring strut wherein transverse forces as low as possible are introduced into the shock absorber of the air spring strut when there is a pivot movement of the lower end of the air spring strut and wherein damage of the resilient member of the air spring is prevented as far as possible.
The air spring strut of the invention is for mounting between a vehicle axle and a body of a vehicle. The air spring strut includes: a shock absorber including a cylinder, a piston rod having a first end and a second end, a piston connected to the piston rod at the first end thereof and being displaceable in the cylinder; an air spring including a cover attached to the vehicle and a roll-off piston supported on the cylinder of the shock absorber; the cylinder having a foot point articulately connected to the axle and the second end of the piston rod being pivotally connected to the cover; the air spring further including a resilient member clamped between the roll-off piston and the cover and the resilient member forming a rolling lobe during operation of the vehicle; the resilient member having an outer surface; a bell enclosing at least a portion of the outer surface of the resilient member; the bell being so configured that each cross section thereof has an approximately circular cross-sectional area defining a center point; and, the center points of all of the cross-sectional areas being disposed along a curve defining approximately a circular path segment over the largest part of the excursion path described by the air spring during deflection thereof.
When the vehicle axle of a motor vehicle contracts and expands relative to the vehicle body, the foot point of the air spring strut, which is mounted between the vehicle axle and the vehicle body, and the roll-off piston of the air spring of the air spring strut describe a circular path segment. The curve, on which the center points of the cross-sectional areas of the bell are arranged, is preferably so selected that it corresponds approximately to the circular path segment which the roll-off piston of the air spring describes for a compressive deflection or expansive deflection of the vehicle axle relative to the vehicle body.
The advantage of the invention is especially that the rolling lobe of the air spring has an approximately rotationally symmetrical cross section in each deflection state of the air spring so that small bending radii of the resilient member, which could lead to damage thereof, are substantially avoided in the region of the rolling lobe. Furthermore, the transverse forces, which are built up within the air spring, are low. For a compressive and expansive deflection of the air spring, the rolling lobe of the air spring assumes an inclined position because of the configuration of the bell in accordance with the invention which encloses the resilient member of the air spring. This is seen as an additional advantage of the invention. Because of the inclined position, the spring force, which is applied by the air spring, runs approximately along the longitudinal axis of the shock absorber of the air spring strut so that no transverse forces are introduced into the air spring strut by the spring force applied by the air spring. In the air spring strut according to the invention, it is a further advantage of the invention that the problem is not present that the roll-off piston of the air spring contacts that part of the resilient member which is braced on the bell of the air spring because the course of the bell corresponds to the path of movement of the roll-off piston. A further advantage of the invention is that a wobbly movable arrangement of the roll-off piston of the air spring on the cylinder of the shock absorber, and therefore the elastic bearing associated therewith, is not necessary whereby the cost of the air spring strut is reduced. A further advantage of the invention is that the roll-off piston of the air spring can be attached at the upper end of the cylinder of the shock absorber so that adequate space for other components is available in the region of the lower end of the cylinder of the shock absorber.
According to another feature of the invention, the diameters of the approximately circular-shaped cross-sectional areas become greater toward the foot point of the air spring strut. The advantage of this embodiment becomes understandable when one considers the following, namely, the bell, which encloses the resilient member of the air spring, can be produced with the aid of a form-imparting core in an especially simple manner (for example, in a casting process) and this core is pulled out after the completion of the bell. The advantage of this embodiment is that the core can be easily removed from the bell when the bell expands toward one end which is achieved with this embodiment.
According to another feature of the invention, the roll-off piston of the air spring is curved. The center curvature line lies on the curve on which the center points of the cross-sectional areas of the bell lie. The advantage of this feature is that the roll-off piston and the bell are optimally adapted to each other and a substantially rotational symmetrical configuration of the roll-off lobe of the resilient member is guaranteed in each deflection state of the air spring.
According to another feature of the invention, the roll-off piston of the air spring is inclined relative to the longitudinal axis of the shock absorber. The advantage of this embodiment will become clear when one considers the following. The circular path section, on which the center points of the cross-sectional areas of the bell lie, has a large radius of curvature and can be approximated by a straight line in a first approximation. The roll-off piston can be positioned inclined relative to the longitudinal axis of the shock absorber so that its center axis runs along this straight line. The advantage of this embodiment is that a simple but nonetheless adequate adaptation of the roll-off piston to the circular path segment is possible because of an inclined position of the roll-off piston. The center points of the cross-sectional areas of the bell lie on the circular path section.